1. Field of the Invention
This invention relates to recording apparatus, such as billing or load survey recorders which are used in the electrical utility field, for recording information relating to power usage on a recording medium, and more particularly, to a power outage indicating circuit for use in such recording apparatus for detecting a power outage condition and recording a power outage indication on the recording medium.
2. Description of the Prior Art
Recording apparatus, such as billing or load survey recorders, are employed by electrical utility companies for measuring electrical parameters and recording data for use in calculating quantities such as power consumption, maximum demand billing, peak load information, and the like over extended periods, typically in the order of a month. One such recording apparatus is disclosed in the U.S. Pat. No. 3,892,772 issued to Norman F. Marsh et al on Aug. 13, 1974. The recording apparatus includes a cassette tape recorder unit having a tape drive which is driven by a synchronous motor energized by power from the AC source being measured to advance a magnetic tape past recording heads, allowing recording of power usage data on tape. The synchronous motor also drives a timing device, embodied as a cam operated time interval switch, to effect the generation of a timing pulse for every fifteen minutes of operation of the motor. The timing pulses control a time record circuit which controls current flow through a time track recording head to record a discrete timing signal on the tape to indicate times of occurrence of the fifteen minute intervals. An NRZ recording format is used for recording time information on the tape. A power outage circuit provides a recorded indication of power interruptions which are in excess of a predetermined duration. The power outage circuit includes a latch circuit which inhibits the time record circuit for a time following restoration of power to prevent current flow through the time track head until the next time the time interval switch operates. This causes two half--amplitude pulses to recorded, with their location and polarity being distinguishable from timing pulses, providing an indication of the power outage.
Other examples of data recorders including power outage circuits are disclosed in the U.S. Pat. No. 3,921,207 which was issued to William P. Doby on Nov. 18, 1975, and in the U.S. Pat. No. 3,913,129 which was issued to Carl J. Snyder on Oct. 14, 1975. In both of these circuits an outage indication is recorded by charging a capacitor following a power outage and the discharging the capacitor over the time record head winding. In normal operation the charging path for the capacitor is interrupted as the result of operation of a relay which is maintained energized as long as power is supplied to the circuit. Upon loss of power, the relay is deenergized and the capacitor is permitted to charge when power is restored. In the circuit shown by Doby, the capacitor charges as soon as power is provided, and in the circuit shown by Snyder, the capacitor charges in response to the operation of an SCR. After restoration of power, the operation of an outage switch allows the capacitor to discharge over the time record head effecting recording of an outage pulse on the tape. The relay is also operated to prevent the capacitor from recharging. The SCR used in the circuit shown in the Snyder patent, isolates the timing capacitor from the power source. The SCR, which is energized by an AC signal, is normally non-conducting and is triggered with restoration of power, and the operation of the outage switch.